It is commonly stated that the diameter of a cylindrical fiber can be calculated from the position of the first minimum in the diffraction pattern using the results of the single‐slit diffraction theory. Calculations show that this approximation works best for the diffraction pattern of a metal wire using light polarized parallel to the axis of the wire. It will yield an error of less than 5% for wires greater than 6 μm in diameter, when the measurement is made with red light. Light polarized perpendicular to the axis gives poorer results, and the approximation is much poorer for transparent fibers.

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